An investigation of the washing of wood pulp fibre suspensions

Abstract

Dissolved organic substances in mechanical pulping, and organic lignin compounds in chemical pulping must be removed to produce pulps of acceptable cleanliness. In chemical processes, the solute-rich liquor is removed by washing, concentrated and burned to recover both the cooking chemicals and energy for the process. Solutes remaining in the washed pulp as a result of an inefficient washing process, increase the cost of bleaching chemicals in subsequent operations or remain as impurities in unbleached pulps. Hence, pulp washing is a vital stage in the overall pulping process, both to maximise solute recovery and to minimise the consumption of fresh wash water. A review of the literature shows that there are several washing mechanisms and many industrial washing processes. Displacement washing is the most strongly favoured for commercial applications. Relatively few results of industrial scale experiments are reported. Most studies have used laboratory-size test cells with stationary pads of pulp fibre. Many of the physical models developed have little application to commercial systems. Process models on the other hand have wide acceptance and usefulness. Some clear trends on the effects of operating variables (pad thickness, fluid velocity, and black liquor concentration) on washing efficiency are presented but there are also some disparate conclusions made, resulting in clear contradictions among authors. Previous research on fibre pad structure and various liquor properties revealed that further work was necessary to capitalise on the trends and the general body of research reported to date. Several areas of need were perceived and this led to further experimental work as reported in this thesis.